Hydrogen bonding: Part 50. IR and thermodynamic study of the hydrates of hexamethonium iodide, bromide, chloride and fluoride

Abstract We have examined the solid hydrates of the dihalides of the hexamethonium dication, (CH 3 ) 3 N + −(CH 2 ) 6 − + N(CH 3 ) 3 , a dipositive analog of simple monopositive quaternary ammonium ions. The iodide forms a dihydrate which shows the IR spectrum associated with linear Type II halide monohydrate hydrogen bonding. At 25°C the bromide forms a dihydrate and a monohydrate; both show IR spectra of Type I planar (H 2 O·Br − ) 2 clusters. Below 49°C equilibrium vapor pressure measurements on the bromide dihydrate give an abnormally high Δ H 0 diss of 34.37 kcal mol −1 . At 49°C a transition occurs to yield a species with an IR spectrum for linear Type II hydrogen bonding and a normal Δ 0 diss of 15.41 kcal mol −1 ; this transition reverses very slowly on cooling. The chloride forms a dihydrate and monohydrate, and possibly a sesquihydrate. Below 36°C the dihydrate shows the IR spectrum of Type I C 2h (H 2 O·Cl − ) 2 clusters; above 40°C the spectrum changes to that of Type II linear hydrogen bonding. The low and high temperature forms of the chloride dihydrate show relatively normal Δ H 0 diss values of 20.95 and 18.18 kcal mol −1 respectively. Both the chloride sesquihydrate and monohydrate give IR spectra of Type I C 2h (H 2 O·Cl − ) 2 clusters; these spectra do not show a transition to Type II on heating. The fluoride forms three hydrates; a hexahydrate, a tetrahydrate and a dihydrate. These hydrates, which are formally a fluoride ion trihydrate, dihydrate and monohydrate, show IR spectra for their water-fluoride ion structures which are identical to those of the corresponding monopositive quaternary ammonium fluoride hydrates, and are assumed to have hydrogen bonding arrangements which are very similar to those found in tetraethylammonium and tetramethylammonium fluoride hydrates. IR observations on the chloride and bromide dihyrdrates are correlated with known crystal structures.